ABSTRACT

Arabidopsis (Arabidopsis thaliana) plants were grown in a hydroponic culture system for 7 to 14 d in the absence or presence of 75 µM Cd or 75 µM Cu. The Cu treatment resulted in visual leaf symptoms, together with anthocyanin accumulation and loss of turgor. Pronounced lipid peroxidation, which was detected by autoluminescence imaging and malondialdehyde titration, was observed in Cu-treated leaves. The Cd treatment also resulted in loss of leaf pigments but lipid peroxidation and oxidative stress were less pronounced than in the leaves exposed to Cu. Analysis of low-molecular-weight chloroplast and cytosolic antioxidants (ascorbate, glutathione, tocopherols, carotenoids) and antioxidant enzymes (thiol-based reductases and peroxidases) revealed relatively few responses to metal exposure. However, there was a marked increase in vitamin E (α-tocopherol) in response to Cd and Cu treatments. Ascorbate increased significantly in Cu-exposed leaves. Other antioxidants either remained stable or decreased in response to metal stress. Transcripts encoding enzymes of the vitamin E biosynthetic pathway were increased in response to metal exposure. In particular, VTE2 mRNA was enhanced in Cu- and Cd-treated plants, while VTE5 and hydroxylpyruvate dioxygenase (HPPD) mRNAs were only up-regulated in Cd-treated plants. Consistent increases in HPPD transcripts and protein were observed. The vitamin E-deficient (vte1) mutant exhibited an enhanced sensitivity towards both metals relative to the wild-type (WT) control. Unlike the vte1 mutants, which showed enhanced lipid peroxidation and oxidative stress in the presence of Cu or Cd, the ascorbate-deficient (vtc2) mutant showed WT responses to metal exposure. Taken together, these results demonstrate that vitamin E plays a crucial role in the tolerance of Arabidopsis to oxidative stress induced by heavy metals such as Cu and Cd.